Genetics: Genetic Material, Central Dogma, and Bacterial Transformation

GENETIC MATERIAL AND ITS FUNCTIONS

• Definition of Genetic Material: Genetic material is the substance present within a cell that controls the characteristics of an organism and ensures the transmission of those characters from one generation to the next. It resides in genes and governs the heredity and expression of parental traits in offspring.
• Essential Functions of Genetic Material:
  • Self-replication: The material must be capable of making exact copies of itself to pass information to the subsequent generation.
  • Information Storage: It must store all necessary information required for the proper physiological and structural functioning of the cell.
  • Variation: The material must be able to undergo changes, known as mutations, to produce genetic variation within a population.
• Chemical Nature: The primary genetic materials in living organisms are chemically classified as nucleic acids:
  • DNA (Deoxyribonucleic acid): Acts as the genetic material in the vast majority of organisms.
  • RNA (Ribonucleic acid): Serves as the genetic material in certain viruses, specifically identified as $RNA$ viruses.

THE CONCEPT AND HISTORY OF THE GENE

• General Definition: A gene is the basic unit of heredity transmitted from one generation to the next, responsible for the inheritance of specific characters.
• Molecular Definition: In molecular terms, a gene is a specific segment of $DNA$ that carries the information required to express a particular trait.
• Historical Background:
  • Term Origin: The term "gene" was coined by the Danish geneticist Wilhelm Johannsen in $1909$.
  • Etymology: Derived from the Greek word "genesis," which means "to be born."
  • Mendelian Context: Before the term gene was established, Gregor Mendel referred to these units as "factors" which transmit individual traits (characters) from parents to offspring.
• Classical Concept of the Gene: Under the classical view, a gene is defined by three specific roles:
  1. Unit of Function: A specific segment of a chromosome controlling the expression of a single trait (e.g., eye color in humans).
  2. Unit of Mutation: The smallest individual segment of a chromosome capable of undergoing a mutational change.
  3. Unit of Segregation (Transmission): The smallest unit of heredity that segregates during meiosis and is transmitted according to Mendel’s laws.

THE CENTRAL DOGMA OF MOLECULAR BIOLOGY

• Definition: The central dogma explains the unidirectional flow of genetic information within a cell, moving from nucleic acids to proteins.
• Standard Flow of Information:
  • The information flows as follows: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$.
  • Transcription: The process where $DNA$ is transcribed into $RNA$.
  • Translation: The process where $RNA$ is translated into a protein (polypeptide).
  • Formulaic Representation: $\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein (polypeptide)}$.
• Original Proponent: This concept was proposed by Francis Crick in $1958$.

REVERSE CENTRAL DOGMA (TEMINISM)

• Definition: In certain viruses known as retroviruses, the genetic material is $RNA$. These viruses follow a reversed flow of genetic information.
• Mechanism: These viruses utilize an enzyme called reverse transcriptase to convert viral $RNA$ into $DNA$.
• The Flow in Retroviruses: Information moves from $\text{RNA} \rightarrow \text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$.
• Chemical Reaction: $\text{RNA} \xrightarrow{\text{Reverse transcription}} \text{DNA}$.
• Discovery:
  • Discovered independently by Howard Temin ($1970$) and David Baltimore ($1970$).
  • The discovery was made while studying the Rous Sarcoma Virus (RSV), which is a retrovirus.

DNA AS THE HEREDITARY MATERIAL: BACTERIAL TRANSFORMATION

• Role of DNA: $DNA$ acts as an information tape encoding instructions for vital activities and transmitting them across generations.
• Definition of Bacterial Transformation: A process where $DNA$ from one bacterial cell is taken up by another, causing the recipient to acquire new traits from the donor.
• Frederick Griffith’s Experiment (1928): Provide the first evidence that $DNA$ carries hereditary information through studies on the pneumonia-causing bacterium Diplococcus pneumoniae in mice.
• Bacterial Strains Identified by Griffith:
  1. R-type (Rough): Non-capsulated and non-virulent (non-pathogenic). It lacks a polysaccharide capsule and forms rough-surfaced colonies. It does not cause pneumonia.
  2. S-type (Smooth): Capsulated and virulent (pathogenic). Each bacterium is enclosed in a polysaccharide capsule, resulting in smooth colonies. It causes pneumonia.
• Experimental Procedures and Results:
  • Trial i: Injection of living $R-type$ bacteria into mice $\rightarrow$ Mice survived.
  • Trial ii: Injection of living $S-type$ bacteria into mice $\rightarrow$ Mice developed pneumonia and died. Living $S-type$ bacteria were recovered from the blood.
  • Trial iii: Injection of heat-killed $S-type$ bacteria into mice $\rightarrow$ Mice survived. No $S-type$ bacteria were found in the blood.
  • Trial iv: Injection of a mixture of heat-killed $S-type$ and living $R-type$ bacteria into mice $\rightarrow$ Mice developed pneumonia and died.
• Critical Observation: In trial iv, despite both components being non-virulent on their own, living $S-type$ bacteria were recovered from the dead mice.
• Conclusion (The Griffith Effect): Griffith concluded that a "transforming factor" from the heat-killed $S-type$ bacteria had converted the living $R-type$ bacteria into virulent $S-type$ bacteria. Although he demonstrated the phenomenon, he did not identify the exact chemical nature of the transforming factor at that time.